This invention relates to optical read-out apparatus and, more particularly, to such apparatus which is capable of reading optically detectable indicia by transmitting and focussing a light beam on the surface of an optically encoded medium and by sensing modulations which are imparted to the light beam by the optically encoded information.
Optical read-out apparatus is used to recover information which has been recorded in an optically detectable format on a record carrier. This information may be represented by analog or digital data which is recorded as optically encoded signals. As examples, this information may be video signal information, audio signal information, binary or digital signal information, and the like. The record carrier may be a rotatable disc, a film strip, a movable card or other record carrier capable of having encoded information recorded thereon. The encoded information is in the form of optically detectable indicia, such as markings which diffract, distort, modulate or otherwise modify some parameter of a light beam which is incident thereon. As a typical example, video signal information may be recorded on a video disc as pits which are provided in substantially circular, concentric tracks, or in a single spiral track, whereby the pits modulate the intensity of a light beam which is transmitted to the record disc.
In optical read-out apparatus for a video disc, a source of light, such as a laser, emits coherent light which is directed and focussed by an optical head to a spot on the surface of the disc. The head generally includes various lenses, mirrors and/or prisms to properly control and shape the beam emitted by the laser. If the video information which is recorded on the disc is in the form of pits, the focussed beam is modulated by such pits and then is reflected to photo-detecting devices which convert the intensity modulations of the reflected beam into corresponding electrical signals. These electrical signals then are demodulated so as to recover the video information which had been recorded on the disc. So-called tracking and time-base errors can be sensed by detecting the reflected beam in order to drive suitable servo control circuits for correcting such errors. In some systems, a separate light beam additionally is directed to the video disc, and reflections of this separate beam are used to determine whether the optical head is in a proper focussing condition. Additional servo control circuitry is provided to maintain a correct focussing condition.
Although the optical head which is comprised of one or more optical lenses and mirrors functions in a generally satisfactory manner, the individual optical elements which constitute this head usually are precise optical instruments and are very expensive to manufacture. In addition, the overall structure of the optical head is relatively complex and expensive to assemble. In some applications, the optical head includes an objective lens which serves the dual function of focussing the laser-emitted light beam onto the surface of the video disc and also collecting the light which is reflected from the disc to transmit the reflected light to the photo-detecting devices. The aperture of this objective lens is small to enhance its focussing of the laser-emitted beam and, consequently, this aperture limits the amount of reflected light which can pass through the lens to the photo-detecting devices. As a result, the sensitivity of the read-out apparatus may be limited.